A Mach-Zehnder modulator (MZM), also known as a Mach-Zehnder interferometer (MZI), is an optical modulator based on interference between two out-of-phase optical signals combined by an optical coupler. In particular, the optical power in each output branch of the optical coupler will depend on the amount of phase difference between the optical signals at the inputs of the coupler.
In order to provide a variable phase difference between the optical signals, it has been proposed to propagate the optical signals through diodes across which a biasing voltage is applied, the phase shift of each optical signal being a function of the applied voltage level. In this way, a voltage signal applied to the diodes can be used to generate a modulated optical signal.
The successful demodulation of the optical signal at the receiver will depend on the extinction ratio of the optical signal, in other words the difference in the light intensity between different levels of the optical signal. In existing MZMs, in order to provide a phase difference between the optical signals that is sufficiently high to achieve a good extinction ratio, it is generally necessary to provide relatively long diodes, for example of between 2 and 3 mm in length. However, the use of relatively long diodes has drawbacks in terms of surface area and energy consumption. Furthermore, while it has been proposed to implement a ring modulator having a feedback path in order to increase the extinction ratio to some extent, it would be desirable to increase the extinction ratio further and/or reduce the energy consumption of such a modulator.
There is thus a need in the art for an optical modulator permitting diodes of reduced length to be used while maintaining a relatively high extinction ratio of the resulting optical signal.